Injection molding is suited to mass-scale production and advantageous for reducing the molding cycle and automatic operation. Molds for use in injection molding of, for example, ring-shaped articles such as rings and frames are generally configured such that a material enters a cavity (defining the article shape) from one gate and is divided into two streams which flow through the cavity and meet again at the position spaced remotest from the gate (where the material reaches latest).
The molds for injection molding have the problem that since their structures are often complex and a material flows in one direction, weld marks are frequently formed at the position where the material reaches latest. In the injection molding of seal members, typically O rings, from rubber material, precise molding is necessary to enhance gas-tightness. It is often required to minimize weld marks. Any strength lowering at weld marks is not permissible.
To improve the strength at the material flow joint and outer appearance of molded articles, the material flow joint, that is, the ultimate site where the material introduced from the gate flows and reaches (i.e., the material reaches latest) is often provided with an air vent for purging the air from the cavity (defining the article shape).
Usually one or more air vents are provided on the cavity surface at the material flow joint. Typically two or more air vents are provided when the material to be molded will undergo a prominent strength lowering at the material flow joint or in the application where any strength lowering at the material flow joint is not permissible.
The air vents are often provided not only for the purpose of purging the air at the material flow joint, but also for the purpose of allowing the partially cured portion of the injected material at its leading end to be discharged out of the cavity (defining the article shape). In the molding of seal members as described above, it is a common practice to purge through the air vents not only the air, but also the leading portion of the injected material.
Particularly in the event where high precision articles such as O rings for use in aircraft are molded, two air vents are provided on the inside and outside of the cavity at the material flow joint where the material injected into the mold reaches latest so that the air at the material flow joint is purged together with the partially cured portion of the injected material at its leading end. The provision of two air vents ensures to make the quality of molded articles consistent.
Meanwhile, fluororubber is an elastomer having excellent heat resistance, chemical resistance and mechanical strength. Seal members, typically O rings, made of fluororubber are industrially employed in a wide variety of applications including automotive and machinery industries.
However, the fluororubber, when injection molded in a mold with a conventional air vent structure, tends to incur a strength lowering at the material flow joint and becomes insufficiently consistent in quality, although high reliability is necessary. For this reason, the manufacture of O rings from fluororubber is often carried out by compression molding rather than injection molding. Undesirably, the compression molding method is incompliant with mass-scale production and low efficient.
Liquid fluororubber compositions as typified by SIFEL produced by Shin-Etsu Chemical Co., Ltd. are expected to find use in O rings. However, they are difficult to produce articles in a consistent manner partly because they are difficult to mold by compression molding, and partly because an optimum mold structure has not been established for the liquid injection molding system (LIMS).